Coverage transition indicator for device-to-device communication

ABSTRACT

A User Equipment (UE) D2D device transmits a coverage transition indicator when the UE device leaves a service area of base station and is no longer in coverage. The coverage transition indicator at least indicates that the UE device has left the service area and is no longer in coverage. Based on the coverage transition indicator, a determination can be made as to whether the D2D group to which the UE device belongs has transitioned to an out of coverage scenario from a partial coverage scenario.

RELATED APPLICATIONS

The present application is a continuation of and claims priority to U.S.application Ser. No. 15/861,341, entitled “COVERAGE TRANSITION INDICATORFOR DEVICE-TO-DEVICE COMMUNICATION” and filed on Jan. 3, 2018; which isa continuation of and claims priority to U.S. application Ser. No.15/105,493, entitled “COVERAGE TRANSITION INDICATOR FOR DEVICE-TO-DEVICECOMMUNICATION” and filed on Jun. 16, 2016; which is a national stageapplication of PCT/US2014/071276, entitled “COVERAGE TRANSITIONINDICATOR FOR DEVICE-TO-DEVICE COMMUNICATION” and filed on Dec. 18,2014; which claims priority to Provisional Application No. 61/919,284entitled “Scheduling for D2D Communications” and filed on Dec. 20, 2013,all assigned to the assignee hereof and hereby expressly incorporated byreference in their entirety.

FIELD

This invention generally relates to wireless communications and moreparticularly to management of device-to-device (D2D) communicationresources.

BACKGROUND

Many wireless communication systems use communication stations (basestations, eNodeBs, eNBs) to provide geographical service areas wherewireless communication user equipment devices (UE devices) communicatewith the communication station providing the particular geographicalservice area in which the UE devices are located. The communicationstations are connected within a network allowing communication links tobe made between the wireless communication devices and other devices.The communication system may include communication stations that provideoverlapping service areas of different sizes to serve various needs ofmobile UE devices. For example, macrocell communications stations mayprovide macrocell service areas that are larger and may cover one ormore small cell service areas provided by small cell communicationstations.

In some circumstances, the communication links are between wirelesscommunication UE devices that are close to each other. In thesesituations, it may be preferred to have a direct communication linkbetween the two wireless UE devices rather than communicating through abase station. Such direct communication between devices is oftenreferred to as device-to-device (D2D) communication or peer-to-peer(P2P) communication. The communication resources (e.g., time frequencyblocks) used for D2D communication are typically a subset of thecommunication resources used by the communication system forcommunication between UE devices and the communication stations.

An in-coverage UE device (InC UE device) is a UE device that is withinthe service area of a communication station and is capable ofcommunication with the communication station. An out-of-coverage UEdevice (OoC UE device) is a UE device that is not within a service areaof any communication station. D2D UE devices that are engaged in D2Dcommunication with each other form a D2D group. A D2D group, therefore,includes two or more D2D UE devices. There are five typical coveragescenarios that occur with D2D groups. In a first coverage scenario, allthe UE devices of the D2D group are located in a service area of asingle communication station. For the first scenario, therefore, all ofthe D2D UE devices of the D2D group are InC UE devices in a singleservice area. In a second coverage scenario, none of the UE devices ofthe D2D group are located inside any service area of any communicationstation. For the second scenario, therefore, all of the D2D UE devicesof the D2D group are OutC UE devices. In a third coverage scenario, atleast one of the UE devices of the D2D group is located in a servicearea of a single communication station and at least one D2D UE device ofthe group is outside all service areas. For the third scenario,therefore, at least one of the D2D UE devices of the D2D group is InC UEdevice in a single service area and at least one UE device is an OutC UEdevice. The third coverage scenario is sometimes referred to as apartial coverage scenario. In a fourth coverage scenario, at least oneof the UE devices of the D2D group is located within a first servicearea of a first communication station and at least one D2D UE device ofthe group located within a second service area of a second communicationstation. For the fourth scenario, therefore, at least two of the D2D UEdevices of the D2D group are InC UE devices in different service areas.In a fifth coverage scenario, at least one of the UE devices of the D2Dgroup is located within a first service area of a first communicationstation, at least one D2D UE device of the group located within a secondservice area of a second communication station, and at least one UEdevice of the D2D group is an OoC UE device. For the fifth scenario,therefore, at least two of the D2D UE devices of the D2D group are InCUE devices in different service areas and at least one D2D UE device ofthe group is an OoC UE device. Other coverage scenarios are possible.Since the UE devices of a group are mobile, there are situations wherethe coverage scenario of a D2D group changes. For example, where onlyone UE device of a D2D group is in coverage and moves out of coverage,the coverage scenario changes from a partial coverage scenario to an outof coverage scenario.

SUMMARY

A User Equipment (UE) D2D device transmits a coverage transitionindicator when the UE device leaves a service area of base station andis no longer in coverage. The coverage transition indicator at leastindicates that the UE device has left the service area and is no longerin coverage. Based on the coverage transition indicator, a determinationcan be made as to whether the D2D group to which the UE device belongshas transitioned to an out of coverage scenario from a partial coveragescenario.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a communication system where macrocellcommunication resources are used for device-to device (D2D)communication.

FIG. 2A is an illustration of the communication system with a D2D groupin an in-coverage scenario.

FIG. 2B is an illustration of the communication system with the D2Dgroup in an out-of-coverage scenario.

FIG. 2C is an illustration of the communication system with the D2Dgroup in an partial coverage scenario.

FIG. 3 is an illustration of the communication system where the D2Dgroup is transitioning from a partial coverage scenario to anout-of-coverage scenario.

FIG. 4A is an illustration of a first stage of a transition of a D2Dgroup from a partial coverage scenario to an out-of-coverage scenariowhere a coverage transition indicator (CTI) is transmitted by a secondto last user equipment (UE) device to leave the service area.

FIG. 4B is an illustration of a second stage of the transition of theD2D group from the partial coverage scenario to the out-of-coveragescenario where D2D timing information is transmitted in response to theCTI.

FIG. 4C is an illustration of a third stage of the transition of the D2Dgroup from the partial coverage scenario to the out-of-coverage scenariowhere a CTI is transmitted by the last UE device to leave the servicearea.

FIG. 5 is a block diagram of a wireless user equipment (UE) device 500suitable for use as the UE devices.

FIG. 6 is a flow chart of a method of performed at an out-of-coverage UEdevice that is communicating within a D2D group that is in partialcoverage.

FIG. 7 is message diagram of communications between UE devices and thebase station.

DETAILED DESCRIPTION

FIG. 1 is an illustration of a communication system 100 where macrocellcommunication resources are used for device-to device (D2D)communication. A communication station (base station, eNodeB, eNB) 102provides wireless communication services to wireless communication UEdevices 104, 106, 108 within a geographical service area 110, sometimesreferred to as a cell. Several communication stations are typicallyinterconnected through a backhaul to provide several service areas tocover large areas. The various functions and operations of the blocksdescribed with reference to the communication system 100 may beimplemented in any number of devices, circuits, or elements. Two or moreof the functional blocks may be integrated in a single device and thefunctions described as performed in any single device may be implementedover several devices. For example, at least some of the functions of thecontroller 112 may be performed by the communication station 102 andvice versa. A cellular communication system is typically required toadhere to a communication standard or specification. TheThird-Generation Partnership Project Long-Term Evolution (3GPP LTE)communication specification is a specification for systems where basestations (eNodeBs) provide service to wireless communication devices(user equipment (UE) devices) using orthogonal frequency-divisionmultiplexing (OFDM) on the downlink and single-carrierfrequency-division multiple access (SC-FDMA) on the uplink. Although thetechniques described herein may be applied in other types ofcommunication systems, the exemplary systems discussed herein operate inaccordance with a 3GPP LTE communication specification.

The communication station 102 is a fixed transceiver station, sometimesreferred to as a base station, an eNodeB or eNB, which may include acontroller in some circumstances. The communication station 102 isconnected to a controller 112 within a network 113 through a backhaulwhich may include any combination of wired, optical, and/or wirelesscommunication channels. For the examples herein, the controller 112includes the functionality of the Mobility Management Entity (MME) andthe Packet Gateway (P-GW). Accordingly, the controller 112 includes ascheduler 114. In the example, the scheduler 114 allocatestime-frequency resources for communication between the wirelesscommunication devices 104, 106, 108 as well as between the base station102 and the wireless communication devices 104, 106, 108. Wireless UEcommunication devices 116, 118 outside of the geographic service area110 are capable of receiving wireless service from the communicationstation 102 when the devices 116, 118 are within the service area 110.Since, the devices 116, 118, however, are out of range of the basestation 102, communication resources cannot be scheduled for the devices116, 118 when they are outside of the service area 110.

The wireless (UE) communication devices 104, 106, 108, 116, 118(collectively UE devices 120) may be referred to as mobile devices,wireless devices, wireless communication devices, and mobile wirelessdevices, UEs, UE devices as well as by other terms. The UE devices 120include electronics and code for communicating with base stations andwith other wireless communication devices in device-to-deviceconfigurations. The wireless communication devices include devices suchas cell phones, personal digital assistants (PDAs), wireless modemcards, wireless modems, televisions with wireless communicationelectronics, and laptop and desktop computers as well as other devices.The combination of wireless communication electronics with an electronicdevice, therefore, may form a UE device 120. For example, a UE device120 may include a wireless modem connected to an appliance, computer,television, or other device.

As explained above, in some situations, two or more UE devices maycommunicate directly with each other without communication through abase station. Such device-to-device (D2D) communication may occurbetween UE devices within the service area of a base station or outsidethe service area. For the example discussed herein, the in coverage(InC) D2D UE devices 106, 108 are within the service area 110 andcommunicate with each other using D2D communication. Out of Coverage(OoC) D2D UE devices 116, 118 are outside of the service area 110 andcommunicate with each other using D2D communication.

The communication station 102 includes a wireless transceiver that canexchange wireless signals with the UE devices 104, 106, 108 within theservice area 110. Transmissions from the base stations and from the UEdevices 104, 106, 108 are governed by a communication specification thatdefines signaling, protocols, and parameters of the transmission. Thecommunication specification may provide strict rules for communicationand may also provide general requirements where specific implementationsmay vary while still adhering to the communication specification.Although the discussion below is directed to the 3GPP Long TermEvolution (LTE) communication specification, other communicationspecifications may be used in some circumstances. The communicationspecification defines at least a data channel and a control channel foruplink and downlink transmissions and specifies at least some timing andfrequency parameters for physical downlink control channels from a basestation to a UE device 120.

A substantial portion of the macrocell communication resources are usedfor transmitting the downlink signals and the uplink signals. Thecommunication station 102 transmits downlink signals to the wirelesscommunication devices 104, 106, 108 using scheduled downlinkcommunication resources of the defined downlink communication resourcesdefined by the communication specification and reserved for downlinkcommunication. The wireless communication devices transmit uplinksignals to the communication station 102 using scheduled uplinkcommunication resources of the defined uplink communication resourcesdefined by the communication specification and reserved for uplinkcommunication. The macrocell communication resources include frequencybands divided in time where each frequency band and segment of time canbe identified by the scheduler 114 and described in control signals sentfrom the communication station 102 to the wireless communication devices104, 106, 108. The communication specifications, or other system rules,therefore, define applicable communication resources for the downlinkand applicable communication resources for the uplink. The scheduler 114allocates different time-frequency resources to different devices toefficiently utilize the resources while minimizing interference.Accordingly, the scheduled macrocell communication resources used forsignals exchanged with one wireless communication device are differentfrom scheduled macrocell communication resources used for other signalsexchanged with other wireless communication devices. As referred toherein, therefore, the defined macrocell communication resources are thecommunication resources reserved for communication by the specificationand/or communication system rules. The scheduled resources for macrocelltransmission for particular signals, however, are a subset of thereserved macrocell communication resources and are typically dynamicallychanged during operation.

In conventional systems, a scheduler allocates time-frequency resourcesfor communication between the communication station and the UE devices.In the examples discussed herein, however, the scheduler 114 and/or abase station controller also at least partially manage time-frequencyresources (D2D communication resources) for communication betweenwireless communication devices 106, 108 to establish and maintain adevice-to-device (D2D) communication link. OoC UE devices, on the potherhand, select resources from reserved OoC D2D communication resourceswithout direct scheduling by the scheduler. An example of D2Dcommunication resource management is provided in patent applicationentitled MANAGEMENT OF DEVICE-TO-DEVICE COMMUNICATION RESOURCES,application no. PCT/US2014/071266, docket Number TUTL 00234PC, filedconcurrently with this application and incorporated by reference herein.

D2D UE devices communicating with each other can be considered to be ina D2D group. In FIG. 1, for example, D2D UE device 106 and D2D UE device108 are in a first D2D group 122 and D2D UE device 116 and D2D UE device118 are in a second D2D group 124. In accordance with at least some 3GPPLTE specification revisions, each D2D group is associated by a uniqueGroup ID.

FIG. 2A, FIG. 2B, and FIG. 2C are illustrations of the communicationsystem 100 for different coverage scenarios of a D2D group 200. The D2Dgroup includes D2D UE device 202, D2D UE device 204 and D2D UE device206. A D2D group includes at least two UE devices. FIG. 2A is anillustration of the communication system 100 for the D2D group 200 foran in-coverage (InC) scenario. All UE devices 202, 204, 206 are withinthe service area 110 for the InC scenario. FIG. 2B is an illustration ofthe communication system 100 for the D2D group 200 for anout-of-coverage (OoC) scenario. All UE devices 202, 204, 206 are outsidethe service area 110 for the OoC scenario. FIG. 2c is an illustration ofthe communication system 100 for the D2D group 200 for a partialcoverage scenario. During the partial coverage scenario, at least one UEdevice 202 is outside the service area 110 and at least one UE device204, 206 is inside the service area.

FIG. 3 is an illustration of the communication system 100 where the D2Dgroup 200 is transitioning from a partial coverage scenario to an OoCscenario. For the example of FIG. 3, a first D2D UE device 202 and asecond D2D UE device 204 are outside of the service area 110 a third UED2D device 206 is transitioning from the service area 110 to outside theservice area 110. In most circumstances, the D2D UE device istransitioning from the service area 110 because the device is moving outof the geographical area where the communication station providesservice.

In order to synchronize D2D communication between D2D UE devices of aD2D group, at least one of the D2D UE devices in the D2D group transmitstiming information. The timing information that a particular UEtransmits may be derived directly from signals and information receivedat the UE device and transmitted by a communication station (e.g.,eNodeB) connected to the network. In some situations, the timinginformation is determined in some other way. For example, if the UEdevice is outside of all coverage areas (service areas), the timinginformation may be determined from the last timing information receivedfrom a communication station when the UE device was in coverage or fromtiming formation received from another UE device. In some situations,D2D UE devices within a D2D group determine which UE device's timinginformation will be used for D2D communication between the UE devices ofthe group. Selection of the UE device that will provide timing for thegroup may be based on several factors. In most situations, the UE devicedetermined to have the closest timing to the system timing is selectedas the UE device that will provide the timing information for the group.For example, if one UE device is within coverage, and other UE devicesof the group are out of coverage, the in-coverage UE device will providethe timing for the group. If all UE devices are out of coverage, theselection of the UE device for providing the timing may be based on thetime elapsed since each UE device was is coverage.

In system operating in accordance with a 3GPP LTE specification (such asRelease-12), the D2D timing information is provided using the D2DSSsignaling. As is known, D2DSS is analogous to PSS/SSS channels that aretransmitted from base stations to UE devices on downlink resources.D2DSS signals, however, are transmitted using SC-FDMA since D2Dcommunication utilizes uplink communication resources. The UE devicesuse the D2DSS to determine frame and subframe boundaries, thereby,determining timing information. The D2DSS resources are available forall D2D data and control channel transmissions. Although a D2D UE hasavailable resources for transmitting the D2DSS as configured by the basestation, however, there may be situations where at least some of the D2DUE devices in a D2D group do not transmit D2DSS (i.e., timinginformation). For example, if the D2D group is in the InC scenario, allD2D UE devices in the D2D group are receiving PSS/SSS and are derivingtiming information from the base station. As a result, there is no needto transmit D2DSS within the group.

In OoC scenarios, the D2D timing cannot be derived directly from systemtiming since none of the UE devices are receiving PSS/SSS from a basestation. At least one D2D UE device in the group transmits D2DSS.

For partial coverage scenarios, the D2D timing is preferably provided bya D2D UE device that derives the timing from a communication stationconnected to the network. More specifically, at least one of the UEdevices within coverage transits D2DSS based on the system timingrelieved via PSS/SSS. When a D2D group transitions from a partialcoverage scenario to an out of coverage scenario, the D2D UE devicetransmitting the timing information can no longer derive the timinginformation from the communication station. In other words, thetransition from partial coverage to out of coverage occurs when the lastUE device remaining in a service area moves out of the service area andcannot receive signals from the communication station. In manysituations, this D2D UE device moving out of coverage would have beenthe D2D UE device providing the timing information since it was the lastD2D UE deriving the timing information directly from the communicationstation before leaving coverage. If the InC UE device is nottransmitting data, however, it typically does not transmit D2DSS. As aresult, the last InC D2D UE device in the service area may not betransmitting the timing information for the D2D group. Some 3GPP LTEspecification proposals and agreements at least contemplate UE devicestransmitting a timing indicator that indicates whether the D2D timinginformation being transmitted by the UE device is derived from basestation timing or in some other way. These timing indicators, however,can only be transmitted if D2DSS is transmitted from the UE device.Where a D2D UE device that is part of D2D group is in coverage but nottransmitting D2DSS, it cannot be determined whether the D2D group is inan OoC scenario or a partial coverage scenario.

For the examples herein, a coverage transition indicator 306 istransmitted from the D2D UE device 206 when the D2D UE device has left aservice area and is no longer in coverage. The indicator 306, therefore,also indicates that the D2D UE device is no longer deriving the timinginformation directly from the base station 102. The coverage transitionindicator 306 is transmitted when the D2D UE device is leaving, or hasrecently left, the service area 110. The coverage transition indicator306 facilitates the determination that all D2D UE devices of the D2Dgroup are out of coverage. In response to the determination, the D2D UEdevices in the D2D group can modify operation by, for example, usingdifferent communication resources for D2D communication.

For the example of FIG. 3, the third D2D UE device 206 is receivingsignals 302 from the communication station 102 that allows the D2D UEdevice 206 to derive the system timing. In systems operating inaccordance with a LTE 3GPP specification, for example, the third D2D UEdevice 206 receives PSS/SSS from the base station 102. The D2D UE device206 transmits D2D timing information 304 to the D2D UE devices 202, 204of the D2D group. The D2D UE device 206 also transmits a timingindicator 304 that allows the other D2D UE devices 202, 204 of the groupto determine when the D2D UE device 206 is or is not deriving the D2Dtiming information 303 directly from the base station 102. For systemsoperating in accordance with some current and proposed 3GPP LTEspecification revisions, the timing indicator 304 is a two bit flag thatis transmitted within the D2DSS of the PD2DSCH. The system timingindicator 304 identifies how the D2D timing information is derived.Examples of characterizations that can be provided by a system timingindicator such as this include indications that the D2D timinginformation is derived from the system timing information, D2D timinginformation is derived from timing information received from another UEdevice, or that the D2D timing information is derived autonomously bythe UE device. Other examples of characterizations include identifyingthe number of timing derivations from system timing. In other words, thetiming indicator may identify the number of UE devices between the basestation and the UE device transmitting the D2D timing information insome cases. Where the D2D timing information is derived directly fromsystem timing, the number of UE devices is zero. Where the D2D timinginformation is derived from D2D timing information provided by anotherUE device that directly derived the D2D timing information from systemtiming, the number of UE devices is one.

A suitable implementation of the second format example of the systemtiming indicator 304 in a system operating in accordance with 3GPP LTEincludes transmitting a two bit flag in the D2DSS where the flag is setto one of four states. For example, a first state (e.g., 00) mayindicate that the D2DSS is derived from PSS/SSS received from a basestation, a second state (e.g., 01) may indicate that the D2DSS isderived from another D2DSS received from another D2D UE device thatderived the D2DSS from PSS/SSS, a third state (e.g., 10) may indicatethat the D2DSS is derived from another D2DSS received from another D2DUE device that did not derive the D2DSS from PSS/SSS, and fourth state(e.g., 11) may indicate that the D2DSS was derived autonomously by theUE device.

Since the LTE specifications only require the transmission of D2DSS whenthe UE device transmits data, the timing indicator 304 is not a reliableindicator of whether any members of a D2D group remain in coverage. Forexample, if a D2D UE of group is in coverage but is not transmittingdata, the UE device does not provide any indication if it remains incoverage. For the examples herein, however, a coverage transitionindicator (CTI) 306 provides information that can be used to determinewhen a D2D group has transitioned from partial coverage scenario to anOoC scenario. The CTI 306 is transmitted when the UE device 206 movesout of coverage. Although the CTI 306 can be transmitted using anysuitable message or signaling, the CTI is transmitted as a one bit flagin the D2DSS of the PD2DSCH in the example herein. Therefore, for theexample, the CTI 306 is transmitted as part of the D2D timinginformation 304.

FIG. 4A, FIG. 4B, and FIG. 4C are illustrations of stages of atransition of a D2D group from a partial coverage scenario to an OoCscenario where CTIs 306 are transmitted by UE devices 204, 206 as the UEdevices 204, 206 exit the service area 110. The example of FIG. 4A-Cshows a D2D group including three UE devices 202, 204, 206 where thegroup transitions from a partial coverage scenario with one UE device202 that is out of coverage (OoC) to an OoC scenario where are all threeUE devices are out of coverage (OoC). In FIG. 4A, the second to last UEdevice 204 to leave the service area transmits a CTI 306 after leavingthe service area. For the example, the CTI 306 is transmitted with aGroup ID identifying the D2D group. The Group ID uniquely identifies theD2D group such that the UE devices 202, 206 in the D2D Group candistinguish CTI transmissions from UE devices of other D2D groups fromCTI transmissions of UE devices within the D2D Group. In somesituations, transmission of the Group ID with the CTI can be omitted.For example, where the system allows for the UE devices to identifytransmissions form D2D group members in some other way, the transmissionof the Group ID may not be necessary. An example of a suitable techniquefor transmitting the Group ID includes transmitting the Group ID withinthe PD2DSCH.

In response to receiving the CTI 304, any InC UE device belonging to theD2D group and that is still in coverage transmits a response that atleast indicates that it is part of the group and still in coverage. Inthe example herein, D2D UE devices remaining in coverage transmit D2Dtiming information such as D2DSS. FIG. 4B is an illustration of theexample where the last UE device remaining in coverage responds to theCTI by transmitting the D2D timing information 304. The D2D timinginformation 304 is transmitted as D2DSS for systems operating inaccordance with some 3GPP LTE specifications. The transmission alsoincludes the Group ID and a timing indicator indicating that the D2DSSwas derived directly from timing information receiving from the basestation such as PSS/SSS. The response is transmitted within a minimumresponse period. An example of a suitable time for minimum time periodis one second from the time the CTI 304 is received at the InC D2D UEdevice 206. The UE devices 202, 204 in the D2D group receive theresponse (D2D timing information) and recognize that the D2D group isstill in a partial coverage scenario.

FIG. 4C is an illustration of the example when the last Inc UE device ofthe group leaves the service area 110 and is no longer in coverage. Aswith the second to last UE device discussed with reference to FIG. 4A,the UE device 206 transmits a CTI 206 indicating that it is no longer incoverage. For the example, the transmission also includes the Group ID.Therefore, the transmission is the same as the transmissions in FIG. 4A.Since, however, no UE devices of the group remain in coverage, noresponse is transmitted from InC UE devices of the group within theminimum time period for a response. When the minimum time period haselapsed and the UE devices 202, 204, 206 do not receive D2D timinginformation 304 that is derived directly from timing informationtransmitted by a base station, the UE devices 202, 204, 206 determinethat the D2D group is in an OoC scenario. In other words, if no D2Dtiming information is received from an InC D2D UE device, the UE devicesdetermine that the group is out of coverage. The UE devices can respondto the determination by adapting communication within the group. For theexample, the UE devices 202, 204, 206 utilize different communicationresources for D2D communication resources, For example, the UE devicesof the D2D group can access a larger pool of resources than could beused when the D2D group was in a partial coverage scenario.

In the related patent application entitled MANAGEMENT OFDEVICE-TO-DEVICE COMMUNICATION RESOURCES, OoC UE devices use a reservedset of OoC D2D communication resources. By applying the principlesherein to such a system, the D2D UE devices in the D2D group thattransitions from partial coverage to out of coverage can detect thetransition using the system timing indicator and transition from InC D2Dcommunication resources to OoC D2D communication resources.

FIG. 5 is a block diagram of a wireless user equipment (UE) device 500suitable for use as the UE devices 202, 204, 206. Each of the UE devicesincludes a transceiver 502 a controller 504, as well as other componentsand circuitry (not shown) such as memory, for example. The transceiver502 includes a transmitter 504 and a receiver 506. The transceiver 502transmits uplink wireless signals to base stations and receives downlinkwireless signals from the base stations. The transceiver can also beconfigured to transmit and receive D2D signals using allocated uplinkcommunication resources. The controller 504 controls components of themobile wireless communication device to manage the functions of thedevice described herein as well as to facilitate the overallfunctionality of the device 500. The controller 504 is connected to thetransceiver 502 and other components such as memory.

FIG. 6 is a flow chart of a method of performed at an out-of-coverage(OoC) UE device that is communicating within a D2D group that is inpartial coverage. The example of FIG. 6, therefore, may be performed bythe UE 202 in FIGS. 4A-4C.

At step 602, the UE device 202 is communicating within the D2D groupwhere the D2D group includes at least one other D2D UE device. At leastone UE device is in the base station service area. For the example, asecond UE device of the group and a third UE device of the group are inthe service area 110 of the base station 102.

At step 604, the UE device 202 determines if a coverage transitionindicator (CTI) has been received from a D2D UE device in the D2D group.If a CTI indicator has been received, the procedure continues at step608. Otherwise, the procedure returns to step 604 and the UE devicecontinues monitoring the appropriate channels for a CTI. As discussedabove, the CTI indicates that the UE device transmitting the CTI hasleft a base station service area and is no longer within coverage.

At step 608 it is determined whether any D2D timing information has beenreceived within a maximum time period from a UE device within the D2Dgroup. The controller 508 executes a timer from the time the CTI wasreceived and determines whether D2D timing information is received froma D2D UE device in the group. As discussed above, the timing informationis transmitted as a D2DSS and includes the Group ID in somecircumstances. If no D2D timing information is received within the maxtime period, the procedure continues and step 614. Otherwise, theprocedure continues at step 610.

At step 610, it is determined that the D2D group is still within partialcoverage. Receipt of the D2D timing information from a UE device withinthe D2D group indicates that the UE device is still within coverage. Asdiscussed above, a UE device of the group that is still in coverage andreceiving PSS/SSS responds to receipt of a CTI from another group memberUE by sending D2D timing information derived from the system timing froma base station (e.g., PSS/SSS). For the example herein, the D2D timinginformation includes, or is otherwise associated with, the Group ID. TheGroup ID can be sent in the PD2DSCH. For the examples, the D2D timinginformation is transmitted with a timing indicator indicating that theD2D timing is derived from system timing from a base station. Asdiscussed above, this information can be transmitted within a 2 bitfield in the PD2DSCH in some implementations

If at step 608 it is determined that no D2D timing information from aD2D Group member has been received within the maximum time period, themethod continues at step 614. At step 614 it is determined that the D2Dgroup is in an out-of-coverage scenario where all D2D UE devices of theD2D group are outside of any service area.

At step 616, the UE device 202 selects resources from a pool of OoC D2Dcommunication resources for D2D communication within the D2D group. Thepool of OoC D2D communication resources is typically includes resourcesin addition to those available for partial coverage scenarios. Since allmembers of the D2D group are outside of base station service areas, theD2D UE devices are less likely to interfere with macrocellcommunications with a base station. In some situations, therefore, theUE devices may use any uplink communication resources afterdetermination that the D2D group is in an 00C scenario. Where the UEdevice 202 is a member of multiple D2D groups, each group is treatedindependently such that the UE member may use OoC D2D communicationresources with one group and more restricted pool of resources for D2Dgroups in partial coverage scenarios.

FIG. 7 is message diagram of communications between UE devices 202, 204,206 and the base station 102. The example discussed with reference toFIG. 7 is an example of messaging for the example of FIGS. 4A-4C andFIG. 6.

At event 702, the UE devices 202, 204, 206 communicate within the D2Dgroup. The down pointing arrow in FIG. 7 illustrates that thecommunication may continue as the other messaging and events of FIG. 7occur. Although D2D communication is not necessarily continuous thethree UE devices remain members of the D2D group. Also, some UE devicesmay not transmit signals during the D2D communication.

At transmission 704, the base station 102 sends timing information suchas a PSS/SSS to the UE devices that are within the service area 110.Since the first UE device 202 begins the example as an OoC UE device,the first UE device 202 does not receive the PSS/SSS. Both the second UEdevice 204 and the third UE device 206, however are in coverage andreceive the PSS/SSS. At event 708, the second UE device 204 leaves theservice area 110. At transmission 710, the base station 102 againtransmits the PSS/SSS. In the interest of brevity and clarity, sometransmissions of PSS/SSS are omitted from FIG. 7. Accordingly, PSS/SSSmay be transmitted more frequently than illustrated in FIG. 7. ThePSS/SSS transmission 710 is not received by the first UE device 202 andthe second UE device 204 since the two devices 202, 204 are out of theservice area 110.

At transmission 712, the second UE device 204 sends a CTI which isreceived by the first UE device 202 and the third UE device 206. Inresponse to the CTI, the third UE device transmits D2D timinginformation such as D2DSS at transmission 714. As discussed above, inthe example discussed, the D2DSS includes the Group ID and a timingindicator indicating that the D2DSS was derived from the PSS/SSS. TheD2DSS is transmitted with the maximum time period from the receipt ofthe CTI.

At event 716, the third UE device 206 leaves the service area 110. As aresult the PSS/SSS at transmission 719 is not received by any of the UEdevices 202, 204, 206. The third UE device 206 send a CTI attransmission 720 which is received at the second UE device 204 and thefirst UE device 202.

At event 722, the UE devices determine that the D2D group is out ofcoverage. Since no D2DSS is received from any D2D group members withinthe maximum time period the UE devices determine that no UE membersremain in coverage. As discussed above, the UE devices may adjust D2Dcommunication within the D2D group by, for example, using differentcommunication resources for D2D communication.

Clearly, other embodiments and modifications of this invention willoccur readily to those of ordinary skill in the art in view of theseteachings. The above description is illustrative and not restrictive.This invention is to be limited only by the following claims, whichinclude all such embodiments and modifications when viewed inconjunction with the above specification and accompanying drawings. Thescope of the invention should, therefore, be determined not withreference to the above description, but instead should be determinedwith reference to the appended claims along with their full scope ofequivalents.

What is claimed is:
 1. A method performed at a device-to-device userequipment (D2D UE), the method comprising: selecting a system timingindicator to be included into a D2D synchronization signal (D2DSS) forsynchronization of D2D communication, from among a first indicator and asecond indicator, wherein the first indicator indicates that the D2DSSis derived from a base station or derived from another D2D UE being incoverage of the base station, and the second indicator indicates thatthe D2DSS is derived from another D2D UE being out of coverage of thebase station; and transmitting the D2DSS including the selected systemtiming indicator.
 2. The method according to claim 1, wherein theselecting comprises selecting the system timing indicator from among thefirst indicator, the second indicator, and a third indicator, and thethird indicator indicates that the D2DSS is derived autonomously by theD2D UE.
 3. A device-to-device user equipment (D2D UE), comprising: acontroller configured to select a system timing indicator to be includedinto a D2D synchronization signal (D2DSS) for synchronization of D2Dcommunication, from among a first indicator and a second indicator,wherein the first indicator indicates that the D2DSS is derived from abase station or derived from another D2D UE being in coverage of thebase station, and the second indicator indicates that the D2DSS isderived from another D2D UE being out of coverage of the base station;and a transmitter configured to transmit the D2DSS including theselected system timing indicator.
 4. The D2D UE according to claim 3,wherein the controller selects the system timing indicator from amongthe first indicator, the second indicator, and a third indicator, andthe third indicator indicates that the D2DSS is derived autonomously bythe D2D UE.
 5. The method according to claim 1, wherein the systemtiming indicator comprises a two bit flag.
 6. The D2D UE according toclaim 3, wherein the system timing indicator comprises a two bit flag.7. An apparatus for controlling a device-to-device user equipment (D2DUE), the apparatus comprising: a processor configured to select a systemtiming indicator to be included into a D2D synchronization signal(D2DSS) for synchronization of D2D communication, from among a firstindicator and a second indicator, wherein the first indicator indicatesthat the D2DSS is derived from a base station or derived from anotherD2D UE being in coverage of the base station, and the second indicatorindicates that the D2DSS is derived from another D2D UE being out ofcoverage of the base station; and transmit the D2DSS including theselected system timing indicator.